A process known as deformation, where subsurface rocks subtly change shapes, is occurring beneath the surface of Yellowstone which alerts experts.

Researchers state deformation occurs when there is a change in the amount of pressure in the magma chamber and experts are keeping an eye on the development.

Seismologists from UNAVCO, a nonprofit university-governed consortium, are using “Global Positioning System, borehole tiltmeters, and borehole strainmeters” to measure minute changes in deformation at Yellowstone.

In an article for the Billings Gazette, David Mencin and Glen Mattioli, geodesists with UNAVCO, say “the strain signal is larger than would be expected if the crust under Yellowstone were completely solid”.

This would suggest lava is flowing, allowing pressure to build in the chamber.

The duo said: “These independent observations agree with other instruments at Yellowstone, like seismometers, that indicate a zone of semi-molten rock starting about three miles beneath the surface.

“We say semi-molten because the entire zone contains only between five and 15 percent liquid rock that occupies small pockets of space between solid rock.”

However, they add “these findings are no cause for alarm”, and these measurements are consistent for a volcano which has been building up for close to a million years.

The Yellowstone Caldera supervolcano last erupted 700,000 years ago but experts say it should blow every one million years or so.

If the Wyoming volcano were to erupt an estimated 87,000 people would be killed immediately and two-thirds of the USA would immediately be made uninhabitable.

The large spew of ash into the atmosphere would block out sunlight and directly affect life beneath it creating a “nuclear winter”.

The massive eruption could be a staggering 6,000 times as powerful as the one from Washington’s Mount St Helens in 1980 which killed 57 people and deposited ash in 11 different states and five Canadian provinces.

If the volcano explodes, a climate shift would ensue as the volcano would spew massive amounts of sulphur dioxide into the atmosphere, which can form a sulphur aerosol that reflects and absorbs sunlight.